A broad spectrum of thermoset epoxide resins is currently being used by the aerospace community for applications on both commercial and military aircraft primarily as composite matrices and adhesives. As a class, epoxies are extremely versatile materials offering such features as excellent mechanical properties, chemical resistance, high adhesive strength, low density, and high electrical insulation. One particular drawback associated with epoxy resins systems, however, is their tendency to absorb moisture which is dependent on both temperature and humidity conditions. It has long been documented that epoxies absorb moisture which causes a disadvantageous plasticization of the material by depressing the polymer glass transition temperature. Many research studies have been devoted to the purpose of gaining an understanding of the effect of moisture absorption on epoxy network structure and resulting property degradation.
The use of coupling agents to bond organic polymers to inorganic surfaces has been known for several decades to improve the chemical resistance to water of the interface bond. In the early 1960's, methylacrylatechrome complexes (Volan.RTM. by DuPont) were proven effective as coupling agents for bonding polyester and epoxy resins to glass fibers to produce composite laminates with improved moisture resistance (P. Yates and J. Trebilcock, SPI 16th Ann. Tech. Conf. Plast. 8-B, 1961). The mechanism of coupling to polyester resins is believed to be due to copolymerization with the methacrylic functions on the chromium (III) chloride complex, but the mechanism for coupling to epoxy resins has not been explained. Complexes of chromium (III) nitrate with certain carboxylic trans-acids have also been prepared for use as coupling agents in the preparation of glass fiber composites (U.S. Pat. No. 3,725,448).
Although the use of chromium complex coupling agents has been successful in reducing moisture absorption at the interface between resin and reinforcement, moisture absorption at the resin surface and into the bulk of the polymer remains a problem. The relatively high water uptake capacity of epoxy resins is due to the presence of --OH groups in the epoxy chains which attract polar water molecules. By the present invention a chromium ion-containing epoxy with improved resistance to moisture is produced wherein the chromium ions are believed to prevent the absorption of water molecules by themselves coordinating to the --OH groups on the epoxy chain. It is anticipated that this improved epoxy formulation will prove useful as a composite matrix resin, adhesive or casting resin for various applications including use in commercial and advanced aircraft fabrication. This improvement has been made without a sacrifice in the mechanical properties of the polymer.
It is therefore an object of the present invention to provide a process for improving the moisture resistance of epoxy resins by the addition of chromium ions.
Another object of the present invention is to provide a process for preparing a lightweight epoxy matrix resin, adhesive, or casting resin with improved moisture resistance for applications on aircraft.
A further object of the present invention is to provide an improved lightweight epoxy resin having improved moisture resistance and useful as an adhesive, matrix resin in composite fabrication or as a casting resin.